A geomark is the recording of a localization on a map. According to Esri ”one Geomark identifies a particular geographical localization, which you wish to save and to reuse later”. A Geomark is equivalent to a GIS feature and can be stored in the .twz file format. That file format store features geometries, attributes, forms, blanks and attachments. Forms are GUI elements used in the PDF world to edit those feature attributes. GeoMark Export lets you export either selected or all visible features from all the feature layers selected and currently, all layers are enabled for editing in the PDF world and any hyperlinks or hot links present in ArcMap become attachments in the .twz file.
The following video talks about a collaboration between Esri ArcGIS and TerraGo using the Geomarks.